Communication networks based on optical signals have significantly greater bandwidth than those based on metallic couplers. An individual optical fiber operating at a single wavelength is capable of transmitting signals at 2.5 Gbit per second or higher. With the advent of wavelength division multiplexing, the capacity of such a fiber can be further increased by combining signals at different wavelengths into a single optical fiber.
To effectively utilize wavelength division multiplexing, narrow band filters are needed for separating the communications at a particular wavelength. The most promising narrow band filter is based on fiber Bragg reflectors. Since fiber Bragg reflectors are well known to the art, they will not be discussed in detail here. For the purpose of the present discussion, it is sufficient to note that a Bragg reflector may be viewed as a grating that has been induced in the core of an optical fiber. The grating consists of periodic alterations in the index of refraction of the core of the fiber. Such alterations may be induced by illuminating the core with a UV light pattern having regularly spaced maxima of sufficient intensity to damage the core. The pattern is typically generated by the interference of two UV light beams. When light having a wavelength twice the spacing of the grating strikes the grating, the light is reflected because of the coherent interference of the various partial reflections created by the alterations in the index of refraction of the fiber core. Reflection filters based on Bragg reflectors are relatively inexpensive.
Unfortunately, reflection filters are less than optimum for optical communications. In optical communication systems a transmission filter is more useful. To convert a reflection filter into a transmission filter, a three port optical circulator is typically utilized. An optical circulator has the property that light entering the n.sup.th port exits via the (n+1).sup.st port. In a transmission filter based on a three port circulator, the reflection filter is connected to the second port. The light signal to be filtered is coupled to the first port. This signal leaves the second port and strikes the reflection filter which reflects light at the reflection wavelength back into the second port. This light then leaves the third port. Hence, the combination of the circulator and the reflection filter is functionally equivalent to a transmission filter connected between the first and third ports of the optical circulator.
While Bragg reflectors are relatively inexpensive, optical circulators are quite expensive. An optical circulator is several times the cost of a Bragg filter. Hence, to reduce the cost of a bandpass filter, less expensive optical circulators are needed.
Prior art optical circulators are constructed from a stack of crystal plates, which are coupled to optical fibers. A significant portion of the cost of a circulator is related to the cost of properly aligning the optical fibers to the stack of plates. Typically, each of the optical fibers must be separately aligned.
The stack of plates usually includes a number of Faraday rotators. Each Faraday rotator requires a separate magnet in these prior art designs. Accordingly, the size of the stack must be increased to make room for the magnets. The size of the plates is also increased by the need to include separate collimating lenses to couple the light from each of the fibers to the stack. The cost of the stack is related to the size of the plates; hence, designs that increase the size of the stack also increase the cost of the circulator.
Another problem with prior art circulators is the degree of isolation provided between the ports. As noted above, light entering the second port of an optical circulator is suppose to exit only through the third port. However, some of the light is routed out of the first port because the Faraday rotators do not function in an ideal manner.
Yet another problem with prior art circulators is the need to have a separate design for each wavelength window. If light differing substantially in wavelength from the design wavelength enters the circulator, the degree of isolation provided is decreased. Further, the fraction of the light entering a port that exits the correct port is reduced if the wavelength of the light is substantially different from the design wavelength. In future wavelength division multiplexed communication systems, a number of channels having different wavelengths will be present at any given time. Hence, a circulator must be able to operate with the necessary isolation and throughput over as wide a range of wavelengths as possible.
A still further problem with prior art optical circulators is the inability to reverse the direction of circulation electrically. In a typical communication network, a number of users communicate with one another over an optical fiber arranged in a loop by sending signals along the fiber in a predetermined direction. For example, in a telecommunications network each subscriber communicates with a central office over a fiber that is arranged in a ring with the subscriber and central office stations disposed along the ring. If the fiber is broken, communication between one or more of the users and the central office will be interrupted. In principle, these users can still communicate with the central office by sending messages along the uninterrupted portion of the loop. However, this requires that the direction of propagation along the fiber be reversed over a portion of the fiber.
Unfortunately, if the fiber ring includes an optical circulator, the direction of circulation of the signals in the circulator must be reversed. That is, light entering port 3 must now exit through port 2, and light entering port 2 must now exit via port 1. Prior art circulators do not include any method for providing this reversal of direction without including a separate switch to reroute the signals. The cost of such switches increases the cost of the optical circulator assembly, and hence, it would be advantageous to provide an optical circulator whose direction of circulation can be reversed by sending an appropriate signal to the circulator.
Broadly, it is the object of the present invention to provide an improved optical circulator.
It is a further object of the present invention to provide an optical circulator with a higher degree of isolation between the ports than prior art optical circulators.
It is yet another object of the present invention to provide an optical circulator that operates over a wider band of frequencies than prior art optical circulators.
It is a still further object of the present invention to provide an optical circulator that is less expensive to fabricate than prior art optical circulators.
It is another object of the present invention to provide an optical circulator in which the direction of circulation may be switched in response to an electrical signal.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.